There are known polymers of conjugated dienes and unsaturated nitrites, i.e. nitrile rubbers. It is also known to hydrogenate these. This improves the heat-aging properties of the polymer. When doing so care is needed to ensure that only hydrogenation of carbon-carbon double bonds occurs. Hydrogenation of the nitrile moieties is to be avoided, as any reduction of the nitrile groups has an undesired and deleterious effect on the properties of the nitrile rubber; in particular it reduces the oil resistance of the nitrile rubber.
It has been proposed to include various additional copolymerisable monomers in nitrile rubbers. Among the copolymerisable monomers mentioned are α,β-unsaturated mono- and dicarboxylic acids. These can be incorporated into the polymer backbone, but difficulty has been encountered when polymers containing carboxyl groups have been hydrogenated. Particularly if the degree of hydrogenation is high, the carboxyl groups have undergone reduction or other side reactions, which has resulted in an unsatisfactory product.
To avoid the problem of hydrogenation of the carboxyl groups it has been proposed to prepare a nitrile rubber composed of a conjugated diene and an unsaturated nitrile, to partially hydrogenate this nitrile rubber and thereafter to add α,β-unsaturated acid; see U.S. Pat. No. 5,157,083. This approach has not proven satisfactory. As the acid is added after the formation of the nitrile rubber the acid moieties are not distributed randomly nor alternately along the backbone of the polymer. Terpolymerisation of a conjugated diene, unsaturated nitrile and unsaturated acid results in a polymer in which the α and β carbon atoms of the acid form part of the main carbon backbone of the polymer. In contrast, polymerisation of conjugated diene and nitrile results in a polymer that has some carbon-carbon double bonds in a vinyl side chain, from 1,2-polymerisation of butadiene, and some carbon-carbon double bonds in the main polymer backbone, from 1,4-polymerisation of butadiene. These double bonds in the main polymer backbone may be in the cis or in the trans configuration. When the polymer undergoes hydrogenation the vinyl groups undergo hydrogenation first, followed by the double bonds in the cis configuration. Hence, the partially hydrogenated polymer to which the α,β-unsaturated acid is added contains mostly or entirely double bonds in the main polymer backbone and in the trans configuration. Reaction with the unsaturated acid results in a product in which the α and β carbon atoms of the acid are not in the main carbon backbone of the polymer. Hence, the chemical structure of a polymer made in this latter way differs from the chemical structure of the statistical polymers that is formed by the terpolymerisation of a conjugated diene, an unsaturated nitrile and an unsaturated acid, where the monomers are statistically or randomly distributed throughout the polymer chain.
European Patent Application No. 933381 is concerned with carboxylated nitrile-group-containing highly saturated copolymer rubber, and in the Background Art discusses three processes for adding maleic anhydride to a nitrile-group-containing highly saturated copolymer rubber. The European application refers to “a highly saturated copolymer rubber”, but it is believed that some degree of unsaturation in the rubber is required, to serve as reaction sites for addition of the maleic anhydride. Disadvantages of all three processes for adding maleic anhydride are mentioned, and it is said that no satisfactory industrial process has been found. Furthermore, the product of the addition, i.e., the maleic anhydride-nitrile-group-containing polymer is said to be unsatisfactory in various properties, including “abrasion resistance and tensile strength which are required for belts and hoses.”
Preparing a carboxylated, hydrogenated nitrile rubber by first preparing a nitrile rubber, then hydrogenating and thereafter adding an unsaturated acid results in an expensive production process. Furthermore, it is difficult to control the amount of acid that adds to the polymer so the quality of the product is uncertain. A product made in this way was introduced commercially but has since been withdrawn from the market.